1. Field of the Invention
The present invention relates to a lockup clutch to be incorporated into a torque converter of automobiles and the like, and more particularly to a friction device to be assembled in such lockup clutch.
2. Description of the Prior Art
Lockup clutches to which the invention is applied are disclosed, for example, in the U.S. Pat. No. 4,240,532, the Japanese Utility Model Publication No. 61-28126, and the Japanese Utility Model Application Nos. 61-170593 and 61-170594.
In such clutches, the piston as the input portion and the driven plate as the output portion are linked in the circumferential direction by means of plural damper springs (compressive coil springs), and the torque vibrations are absorbed by the expansion and contraction of these springs.
The torque vibration absorbing characteristics of clutches depend, generally, not only on characteristics of damper springs but also on friction characteristics of discs. That is, when the piston and driven plate of the lockup clutch deflect relatively, it is composed to cause a proper friction corresponding to such deflecting action, thereby building up a proper hysteresis torque on the deflection characteristics of the disc, so tha the torque vibrations may be effectively absorbed.
In the conventional lockup clutches, however, since there is no particular mechanism for generating the friction, the hysteresis torque cannot be set accurately to a desired value.